Hi Tim! Actually it's possible to do this without losing data. You just need a tool like Mini Tool Partition Wizard (you can use it on Windows or you can use the bootable version). I've converted a partion from extended to primary using Minitool even on Windows and it worked perfectly!
I've finally undertood that Win7 32 bits just support mbr disks+ bios, while Win7 x64 support also gpt disk + uefi.wrote: Do you know if 7 is able to boot on GPT and UEFI
The problem is that Win7 always "force" itself to boot in bios uefi. To be sure to boot it in uefi mode you have to "start" the "bootx64.efi" file that it's present in the iso. (Very often it's "hidden" in some sub-directory of the iso or you have to extract it from the .wim file)
So, finally I reinstalled MX18 in pure uefi mode alongside win7 on my GPT disk.
I created a fat32 Efi partition with the "boot" and "esp" flags and I used the MX installer to install grub on this partition.
Booting MX manually selecting "bootx64.efi" works, but when I try to boot the computer "normally" I get this
No bootable image found, notebook will be shutdown